1. Field of the Invention
The present invention relates to: a cyan ink containing a water-insoluble coloring material; and an ink set having the same. More specifically, the present invention relates to a cyan ink suitable for an ink-jet recording method.
2. Related Background Art
Ink containing, as a coloring agent, a water-insoluble coloring material such as a pigment (pigment ink) has been conventionally known to provide an image excellent in fastness such as water resistance and light resistance. Various techniques have been recently proposed for the purpose of increasing the image density of an image formed by means of such ink.
For example, there has been proposed the use of ink containing self-dispersion carbon black and a specific salt to achieve an additional increase in image density (see, for example, Japanese Patent Application Laid-open No. 2000-198955). A technique has also been proposed, which involves applying onto a recording medium an ink-jet recording ink, which is a composition containing a pigment, polymer fine particles, a water-soluble organic solvent and water as well as a polyvalent-metal-salt-containing aqueous solution; and causing the ink composition and the polyvalent-metal-salt-containing aqueous solution to react with each other to form a high-quality image (see, for example, Japanese Patent Application Laid-open No. 2000-63719). In each of those techniques, a pigment present in ink in a dispersed state is forcedly flocculated at the surface of a recording medium to suppress the permeation of the pigment into the recording medium, thereby obtaining an image with an increased density.
The investigation by the inventors of the present invention has revealed that each of the above techniques may not provide a sufficient area by which the surface of a recording medium can be covered with a coloring material relatively to the volume of an ink droplet (so-called an area factor) because pigment particles are flocculated on the recording medium. This finding means that each of the above techniques requires an increased amount of ink to be applied as compared to that of conventional pigment ink obtained by dispersing a pigment by means of a polymer dispersant or the like for obtaining the same image density, and each of the above techniques is susceptible to improvement in this respect. There are methods of obtaining a large area factor even with an ink droplet having a small volume by improving the permeability of ink into a recording medium. However, when the permeability of ink is improved, the ink permeates into not only the surface of a recording medium but also the depth direction of the recording medium, so sufficient image density may not be obtained.
The inventors of the present invention have pursued the advantages and disadvantages of the respective conventional inks and analyzed the characteristics of an image itself. As a result, they have revealed that, when a coloring material is present in ink in a high concentration, a large amount of excessive coloring material is present on the surface of a recording medium, or dots having visually different shapes are formed, and that a waste coloring material not involved in color development is present in the recording medium.
When one attempts to increase the coloring material content of cyan ink to increase image density, in an image formed on, for example, a recording medium having surface gloss, in particular, a phenomenon in which metallic luster occurs depending on the angle at which the image is seen (bronzing) occurs. The bronzing has been found to occur more remarkably when a larger amount of pigment flocculates is present on the surface of a recording medium.
A reduction in coloring material content of cyan ink is expected to suppress the bronzing, that is, to obtain excellent bronzing resistance. However, the following has been found. When cyan ink is used in combination with any other ink to form an image, a reduction in coloring material content of only the cyan ink may reduce the image density of an image to be formed by the cyan ink or may affect the color tone of an image to be formed by color mixing of the cyan ink and other ink such as green ink or blue ink, so an image may be poor in color balance.
The inventors of the present invention have found that an image superior to a conventional one can be formed by solving at least one of the above technical problems. The present invention solves at least one of the following problems found by the inventors of the present invention.    (1) A problem in which, when a pigment present in ink in a dispersed state is forcedly flocculated at the surface of a recording medium, an area by which the surface of the recording medium can be covered with a coloring material (so-called an area factor) may not be sufficient relatively to the volume of an ink droplet, so the amount of ink to be applied necessary for obtaining the same image density increases.    (2) A problem in which, when the permeability of ink is improved, the ink permeates into not only the surface of a recording medium but also the depth direction of the recording medium, so a coloring material cannot be distributed in a high concentration near the surface of the recording medium, and hence a high image density cannot be achieved.    (3) A problem in which, when an image is formed on a recording medium having surface gloss by means of an ink set having at least four kinds of aqueous inks composed of cyan ink, magenta ink, yellow ink, and black ink, each of which has an increased coloring material content for the purpose of increasing the image density on a recording medium such as plain paper, through color mixing of the inks of respective colors, bronzing occurs at an image portion formed by the cyan ink. A problem in which an image is poor in color balance when the coloring material content of the cyan ink in the ink set is reduced for suppressing the bronzing.